The classic radiograph or "X-ray" image is obtained by situating a target to be imaged between an X-ray source and an X-ray detector made of photographic film. Emitted X-rays pass through the target to expose the film, and the degree of exposure at the various points on the film are largely determined by the density of the target along the path of the X-rays.
It is now common to utilize solid-state digital X-ray detectors, e.g., an array of switching elements and photodiodes, in place of film detectors. The charges generated by the X-rays on the various points of the detector are read and processed to generate a digital image of the target in electronic form, rather than an analog image on photographic film. Digital imaging is advantageous because the image can later be electronically transmitted to other locations, subjected to diagnostic algorithms to determine properties of the imaged target, and so on.
In both analog and digital radiographic imaging systems, there is commonly a need to properly position the target relative to the X-ray source and the X-ray detector. This is particularly true for cases where the physical dimensions of the target approach the dimensions of the X-ray detector. For the sake of efficiency, it is desirable to center the imaging apparatus so that the image is centered about the area of interest on the target, and it is also desirable to collimate the emitted radiation to cover only so much area on the target and its surroundings as necessary. This is particularly true in the case of digital imagers, wherein uncollimated radiation which misses the target may strike the detector to saturate it. This can potentially disable the digital detector for extended periods of time.
In order to overcome the aforementioned problems, centering of the target is generally accomplished by the use of a light source on the X-ray source. The light source is collimated to project a beam of light which is generally coincident with the radiation beam. Thus, a technician may adjust the location of the target within the light beam, and/or collimate the radiation and light beams with respect to the target, so as to center the target as desired. However, targeting errors may still occur owing to difficulties in determining the light beam's precise boundaries, and similar factors. There is thus a need for improved targeting apparata and methods.